You can't just throw in any old feature phone into the mix though. To be eligible, your phone has to be no more than three years old, and in good, working condition AT&T said.

How it works

On its Trade-in Program website, AT&T will ask you the device model and manufacturer in order to determine its value. You then have to go through a series of questions on the device condition, including screen and body damage, and include your phone's serial number. This process will get a price quote for the phone.

In the next step, you print out a prepaid mailing label to use and ship the phone to AT&T for free. Then, the wireless carrier says, you will receive a promotion card in up to four weeks to use towards the purchase of a new device. Alternatively you can go to an AT&T store to recycle your old smartphone, and you will be able to get the voucher on the spot.

If your trade-in is valued at more than $100, you will receive a credit for the larger amount.

AT&T estimates that most phones it will get through the program have a value of around $100, so it can offer new devices like the Samsung Galaxy S4, BlackBerry Z10, or the HTC One for $99 with a two-year contract instead of the usual $199.

It isn't clear whether you will be able to use more than one recycled old smartphone to subsidize the cost of a new one. We reached out to AT&T about this but have not yet heard back.

Get a second valuation

AT&T is no stranger to phone-recycling programs. Last year the carrier recycled more than 3.1 million phones and broke the Guinness World Record for cell phones recycled in a one-week period.